1. Field of the Invention:
This invention relates to an apparatus for making biaxially stretched tubular films of thermoplastic synthetic resin in a stabilized manner and by using a heat source effectively.
2. Description of the Prior Art:
As for an apparatus for making biaxially stretched tubular films of thermoplastic synthetic resin, there is generally known a process in which an unstretched raw film delivered by pinch rolls is heated up to a stretching temperature by way of various methods, filling a gas having a pressure sufficient to inflate said raw film sealingly, into the inside thereof and taking up said film with stretching pinch rolls having a circumferential speed faster than that of delivery pinch rolls.
As for heating apparatus for a raw film, an infrared radiation system or a hot air circulation system, and as for cooling after stretching, a water cooling system and an air cooling system have been known.
In a stretching apparatus which depends on the inside pressure of a bubble, when an unstretched raw film is heated and reaches a high temperature, its tensile strength decreases and it starts inflation in a circumferential direction by the pressure of gas within the bubble, and at this point, if the temperature is suitable to stretching, the film is stretched and increases its tensile strength and inflation stops at a point where the tensile strength has exceeded the expansion power caused by the inside pressure of the bubble. It has been also known that there is a process in which inflation is regulated by elevating the tensile strength by way of the forced cooling of an inflating bubble. In carrying out biaxial stretching by using these apparatuses in a stabilized manner, uniform heating of a raw film, the uniformity of temperature at the stretching part and the uniformity of temperature at the stretching-terminating part are indispensable. According to conventional apparatuses, even if the uniform heating of a raw film and the uniformity of temperature at the stretching part are maintained, there are drawbacks in that the uniformity of temperature at the stretching-terminating part and the stability of ambient atmosphere cannot be controlled, and the quality and the form of a bubble are unstable by the influence of temperature variation and flow variation of air inside a room where said apparatus is operated.